1. Field of the Invention
Embodiments relate to wireless local area networks (WLANs) and, in particular, to a system and method for calculating the distance between two wireless devices communicating over a WLAN.
2. Description of the Related Art
Global Positioning System (GPS) technology—or other forms of radio resource measurement (RRM) technology—can be used to calculate an accurate geographical location of a mobile device. In some cases, however, the area in which the mobile device is located can cause these technologies to produce inaccurate results. This occurs, for example, in areas where signals from GPS satellites are blocked or overly attenuated, such as in downtown city centers with large buildings, mountainous regions, and indoor environments. In such areas, RRM-based measurements suffer from the same inaccuracies as the GPS-based measurements. Additionally, consumer-grade GPS-enabled devices (e.g., smart phones) may exhibit low-accuracy location calculations even in open areas where accurate results are expected to be obtained, since the GPS receivers in such devices have lower sensitivity than high-quality state-of-the-art GPS receivers included in, for example, professional and military grade mobile devices.
One useful metric pertaining to wireless device location calculations involves determining a distance between two wireless devices, such as an access point and a mobile device communicating therewith. The majority of existing techniques directed toward distance measurement between two devices are based on received signal power. However, received signal power is a non-linear function of the distance between the wireless devices, and is both time-variant and highly dependent on the environment in which the wireless devices are communicating. This produces inaccurate distance calculations. Moreover, in implementations that attempt to calculate distance based on received signal power, routine calibration of the environment is required to be performed periodically, which increases overhead and complexity.
Another technique for distance measurement between two wireless devices employs round-trip-time (RTT)-based measurements that attempt to estimate the time occurring between the transmission of data and the subsequent reception of an acknowledgement signal (ACK). Since RTT is a linear function of the distance of the transmission, RTT-based distance measurements can outperform the accuracy of distances that are calculated according to received signal power. However, most RTT-based distance measurements are calculated from the beginning of the data transmission until the entire ACK signal is received. In particular, the distance measurement is based on a premise that hardware (HW) interrupts trigger at both the start time of the data transmission and at the time the media access control layer (MAC) ACK is completely decoded. One problem with this approach is that HW interrupts are handled by a mobile device's underlying operating system (OS) and, as a result, are not always precisely timed, which causes the HW interrupts to vary significantly based on the number of interrupts scheduled and the overall central processing unit (CPU) workload. Consequently, high jitter signals can be introduced in the distance measurements, which may increase when, for example, other processes consume CPU resources or when a power saving mode of the CPU is activated.
As the foregoing illustrates, there is a need in the art for an improved technique for calculating a distance between two wireless devices communicating over a WLAN.